PRF1 and COVID-19: To determine the influence of Omicron BA.2 infection after COVID-19 vaccination on NK cell activity and cytotoxicity, we further evaluated the expressions of cytotoxic mediators, perforin and granzyme B. Additionally, the production of IFN-γ was also evaluated via intracellular cytokine staining after a short-term restimulation of NK cells in vitro using flow cytometry.